1. Field of the Invention
The present invention relates to a camera provided with a cam member for interlocking an image-taking optical system and a view finder optical system.
2. Description of the Related Art
A view finder optical system which is provided in a camera having a collapse type lens barrel is conventionally designed to interlock with a magnification-varying operation of an image-taking optical system and there are various types of the interlock mechanism; a type of driving a view finder optical system by a cam through a transmission member such as a gear train from a driving mechanism of the image-taking optical system or a type of driving a view finder optical system by an actuator different from the actuator for driving the image-taking optical system while detecting the focal length of the image-taking optical system.
However, in the interlock mechanism which drives the view finder optical system through the transmission member, the transmission member must be provided outside the lens barrel for structural reasons, therefore it is difficult to reduce the size of camera. Furthermore, in the interlock mechanism using a different actuator, the number of parts making up the driving mechanism of the image-taking optical system and the view finder optical system is increased, which constitutes major detrimental effects on not only miniaturization of the camera but also costs.
As means for solving these problems, there is a mechanism which interlocks an image-taking optical system and view finder optical system through a platy cam member (view finder cam plate) as disclosed in Japanese Patent Application Laid-Open No. 2001-324749. That is, this mechanism intends to reduce the number of parts and reduce the size of the camera by interlocking the image-taking optical system and view finder optical system using the platy view finder cam plate.
The view finder cam plate adopts a structure shown in FIG. 17. That is, a view finder cam plate 46 rotates around the optical axis (moves in the horizontal direction in FIG. 17) according to a magnification-varying operation of an image-taking optical system and lens holding members 56 and 57 which hold lenses making up the view finder optical system move in the direction of the optical axis (vertical direction in FIG. 17) through an engagement of follower portions 56a and 57a and cam groove portions 46c and 46d. 
In this interlock mechanism, when the lens barrel moves between a collapsed state and an image-taking state (wide-angle state), the driving force of the lens barrel is not transmitted to the view finder cam plate 46. Then, when the lens barrel moves between the wide-angle state and the telephoto state, the view finder cam plate 46 is driven.
For this reason, it is possible to reduce the driving range of the view finder cam plate 46 compared to the case where the cam member is moved according to the movement of the lens barrel between the collapsed state and telephoto state and thereby reduce the size of the camera.
Here, the view finder cam plate 46 is energized leftward in FIG. 17 by a tensile spring 47 and when the lens barrel moves from the wide-angle state to the telephoto state, the view finder cam plate 46 is driven rightward in FIG. 17 against the spring force of the tensile spring 47 (force indicated by arrow B in FIG. 17).
In the interlock mechanism using the above described view finder cam plate 46, the lens holding members 56 and 57 are energized by a tensile spring 59a in a direction in which both lens holding members come closer to each other as shown in FIG. 17 and pushed to one side by the follower portions 56a and 57a contacting the inner wall surfaces of the cam groove portions 46c and 46d. 
At this time, the force generated by the tensile spring 59a is transmitted to the cam groove portions 46c and 46d through the follower portions 56a and 57a of the lens holding members 56 and 57 and a force is generated rightward (force indicated by arrow F) in FIG. 17 in the view finder cam plate 46. Here, the force given by the follower portion 56a to the side of the cam groove 46c is indicated by arrow A3 and the force given by the follower portion 57a to the side of the cam groove portion 46d is indicated by arrow A4.
On the other hand, the view finder cam plate 46 is energized leftward in FIG. 17 by the tensile spring 47 as described above and two forces in opposite directions (forces indicated by arrow B and arrow F) act on the view finder cam plate 46.
For this reason, the force (arrow B) by the tensile spring 47 which moves the view finder cam plate 46 in one direction (leftward in FIG. 17) is canceled out by the amount of the force (arrow F) with which the tensile spring 59a moves the view finder cam plate 46 in another direction (rightward in FIG. 17).
Here, when the view finder cam plate 46 is driven from the telephoto state to the wide-angle state, the view finder cam plate 46 is driven by only the spring force of the tensile spring 47. However, the part of the one-directional moving force of the view finder cam plate 46 by the tensile spring 47 is canceled out by the another-directional moving force of the view finder cam plate 46 by the tensile spring 59a and therefore the one-directional moving force of the tensile spring 47 is weakened by the amount of the force canceled out, which may repress the view finder cam plate 46 from being driven up to the wide-angle state.
As a solution to this problem, the force of the tensile spring 47 may be strengthened, but in this case, a large driving force is required when the view finder cam plate 46 is driven against the spring force of the tensile spring 47 and, for example, a larger amount of current needs to be supplied to the driving source.